A DOUBLE-QUANTUM SOLID-STATE NMR TECHNIQUE FOR DETERMINING TORSION ANGLES IN POLYMERS

This paper introduces a double-quantum solid-state NMR experiment for determining torsion angles in unoriented polymers which contain segmen ts with pairs of C-13-labeled sites separated by only one or two bonds . By double-quantum excitation and evolution, two-dimensional spectra are obtained in which the first dimension displays the sum of the anis otropic chemical shifts. As a consequence, the two-dimensional spectra l patterns depend on the relative orientation of the coupled sites and thus on the torsion angle. The double-quantum approach achieves homon uclear dipolar decoupling in the first dimension without multiple-puls e sequences and removes the natural-abundance background signal. The e xperiment is demonstrated on polyethylene isotopically labeled with di lute (similar to 4%) C-13-C-13 spin pairs and confirms the all-trans s tructure in the crystalline regions. In polypeptides, it will be appli cable to the determination of the backbone torsion angle psi in a doub ly C-13-labeled amino acid residue, with a sensitivity to analyze one residue in a repeat unit comprising up to 20 residues.